Method of imaging a 2D sample with a multi-beam particle microscope

ABSTRACT

A fast method of imaging a 2D sample with a multi-beam particle microscope includes the following steps: providing a layer of the 2D sample; determining a feature size of features included in the layer; determining a pixel size based on the determined feature size in the layer; determining a beam pitch size between individual beams in the layer based on the determined pixel size; and imaging the layer of the 2D sample with a setting of the multi-beam particle microscope based on the determined pixel size and based on the determined beam pitch size.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. application Ser. No. 16/734,741, filed on Jan. 6, 2020, whichclaims benefit under 35 U.S.C. § 119 to German Application No. 10 2019000 469.8, filed Jan. 24, 2019. The contents of this application ishereby incorporated by reference in its entirety.

FIELD

The present disclosure relates to charged particle beam systems andmethods. More particularly, the present disclosure relates to a methodof imaging a 2D sample with a multi-beam particle microscope, acorresponding system and a corresponding computer program product. Thepresent disclosure is particularly suited for reverse engineering ofintegrated circuits.

BACKGROUND

Single-beam particle microscopes have been known for a long time. Inthese, a single beam is focused via particle optics onto an object to beexamined and scanned over the latter. The particle beam can be an ionbeam or an electron beam. Secondary particles, such as e.g. electrons,emitted from a location where the particle beam is incident, aredetected and the detected particle intensity is assigned to thelocations of the object on which the scanning particle beam is currentlydirected. Thus, it is possible to generate a particle-optical image ofthe object. Scanning of a field of view of a particle microscope withthe particle beam involves time. The extent of the field of view islimited. If relatively large parts of the object are intended to bescanned, the object is desirably moved relative to the particlemicroscope to scan further fields of view. This in turn involves time.There is a desire for particle microscopes that can scan many objectsand relatively large objects in a shorter time. It is conceivable toprovide a larger number of single-beam particle microscopes, themicroscopes operating in parallel to scan a plurality of objectssimultaneously. However, this is a very expensive solution since adedicated particle microscope with particle optics would be provided foreach individual particle beam.

Here, multi-beam particle microscopes form a promising approach since aplurality of particle beams is guided jointly through a single particleoptics arrangement in order to simultaneously scan the object to beexamined with a bundle of particle beams. A multi-beam charged particlebeam system is disclosed, for example, in WO 2005/024881 A2 and in WO2016/124648 A1.

A possible application of single-beam particle microscopes as well as ofmulti-beam particle microscopes is the structure analysis of 3D samples,in particular reverse engineering. Reverse engineering, also called backengineering, is the process by which a man-made object is deconstructedto reveal its designs, architecture, or to extract knowledge from theobject. For the structure analysis of 3D samples, an imaging process anda delayering process can be combined in a workflow. Imaging of the 3Dsample is then done layer by layer. The data gained by imaging acomplete stack of layers allows reconstructing a 3D data set of the 3Dsample.

However, when high resolution is involved in imaging, to achieve forexample a voxel size in the nanometer regime, huge amounts of data areusually collected. When a single-beam particle microscope is used forimaging, reverse engineering of an integrated circuit can take, forexample, several months. Even with a multi-beam particle microscope, thetime needed for reconstruction is still comparatively high and typicallystill can take several weeks. Therefore, a general challenge is to speedup imaging and therefore 3D sample reconstruction.

For integrated circuits, however, the feature size within the integratedcircuits typically varies with the depth inside the 3D sample: thesmallest features that involve highest resolution scanning are at thebottom layer of the chip, with the feature size in the layers aboveusually increasing gradually.

In this context, an approach to speed up the imaging process using asingle-beam particle microscope can be adapting the scan pixel size to avalue that is suitable for imaging the expected minimum feature size inthe layer currently scanned. However, the overall speed for the 3Dreconstruction of a 3D sample is still too low.

SUMMARY

The present disclosure seeks to provide a faster method for imaging a 3Dsample with a multi-beam particle microscope. The method shall beparticularly suited for reverse engineering of 3D samples and inparticular for reverse engineering of integrated circuits.

According to a first aspect of the disclosure, the disclosure isdirected to a method of imaging a 3D sample with a multi-beam particlemicroscope, the method including the following steps:

providing a layer of the 3D sample; determining a feature size offeatures included in the layer;

determining a pixel size based on the determined feature size in thelayer; determining a beam pitch size between individual beams in thelayer based on the determined pixel size; and imaging the layer of the3D sample with a setting of the multi-beam particle microscope based onthe determined pixel size and based on the determined beam pitch size.

Adjusting a pixel size for an imaging process using a single-beamparticle microscope is a key feature for speeding up the imagingprocess. However, investigations of the inventors have shown that asimple transfer of that concept to a multi-beam particle microscope doesnot directly translate to the desired speed up. If the number of pixelsper field of view becomes small, the image acquisition time for thisrespective field of view becomes small. However, the data acquisitiontime for an image containing multiple fields of view will then bedominated by overhead times for stage movements that are desired afterevery imaging of a field of view. Throughput calculations withcalibrated speed data have shown that there are almost no speed gains byincreasing the pixel size beyond a certain limit when using a low speedstage. A solution is to provide a faster stage, but this is veryexpensive. Therefore, the present disclosure provides a higherthroughput even with a low speed stage but even more with a high speedstage:

According to the present disclosure, a beam pitch size between theindividual beams of the multi-beam particle microscope in the plane ofthe layer to be imaged is determined based on the determined pixel size.Thereafter the beam pitch size between the individual beams of themulti-beam particle microscope in the plane of the layer to be imaged isadjusted to the determined beam pitch size and images of the layer arerecorded with the multi-beam particle microscope with the adjusted beampitch size between the individual beams. This has the effect that thespeed up achieved by choosing an appropriate pixel size and moreparticularly in reducing the number of pixels to be acquired can bekept.

In the following, the disclosure will be described in more detail.

Preferably, imaging a 3D sample involves collecting a 3D data set of thesample. The dimensions of the sample itself can nevertheless be verysmall, for example in the micrometer regime. Preferably, the 3D samplecan be sliced or divided into a plurality of layers. The 3D sample assuch can be of any type. Examples are neurobiological samples orintegrated circuits. In both examples, high resolution imaging isinvolved since the voxel size which is the size of the 3D data “point”is typically in the nanometer regime.

The charged particles with which a multi-beam particle microscope isoperated can be for example electrons, positrons, muons, ions or othercharged particles.

The method according to the present disclosure includes providing alayer of the 3D sample. This means that a layer of the 3D sample isprovided in such a manner that its surface can be imaged using themulti-beam particle microscope. Two general approaches can be taken toprovide the layer of the 3D sample: according to a first approach, the3D sample is delayered in a non-destructive manner. Here, it is forexample possible to cut or slice the 3D sample into a plurality oflayers wherein each layer is put onto a substrate and can then beindividually investigated with the multi-beam particle microscope. Asecond approach is destructive delayering of the 3D sample. According tothis approach, the surface of the 3D sample is imaged first. Afterwards,this surface is removed, for example by ion beam milling, ion beamsputtering or charged particle beam induced gas etching, so that the newsurface generated by delayering embodies the next layer that can beimaged. Technical examples for delayering of the 3D sample will be givenbelow in this application.

According to the present disclosure, a feature size of features includedin the layer to be imaged is determined. Determining a feature size canfor example be realized by measuring a feature size of features includedin the layer of interest. Alternatively, there already exists priorknowledge, e.g. design data or other information, about feature sizes offeatures included in the layer to be imaged. According to a preferredembodiment of the disclosure, the determined feature size is a minimumfeature size in the layer to be imaged. The minimum feature sizeinvolves the highest resolution during imaging. The minimum feature sizetherefore limits the overall pixel size and therefore imaging speed forthis layer of the 3D sample. Alternatively, the determined feature sizecan be an average feature size if highest resolution imaging is notneeded. Imaging a 3D sample with a multi-beam particle microscopeinvolves scanning with all beams in two different directions, inparticular orthogonal directions. According to an embodiment, thefeature size in the layer of interest is determined for each imagingdirection separately. The pixel size needed to adequately scan thesmallest features in each of the directions can vary in the twodirections. Determining feature sizes in both directions in a separatemanner therefore contributes to finding an optimum pixel size andtherefore image acquisition speed. According to another embodiment, theminimum feature size is determined independent from imaging directions.

According to the present disclosure, a pixel size is determined based onthe determined feature size in the layer to be imaged. The pixel sizedefines the resolution of imaging. The pixel size can vary in differentimaging directions, but it is also possible that the pixel size isidentical for both imaging directions. According to a preferredembodiment, the determined pixel size for imaging a respective layer isan optimum pixel size. This means that the pixel size is determinedstill small enough to cover the feature to be measured with a sufficientnumber of pixels, and it is at the same time big enough to allow for themaximum throughput. According to a preferred embodiment, a relationbetween the minimum feature size fs_(min) and the optimum pixel sizeps_(opt) is ps_(opt)≤0.5 fs_(min). With a sufficiently small setting,the smallest features can be imaged faithfully. Preferably, the aboverelation holds for feature sizes and pixel sizes in all directions.Preferably, at least one of the following relations is fulfilled: 0.1fs_(min)≤ps_(opt)≤0.5 fs_(min), or 0.2 fs_(min)≤ps_(opt)≤0.5 fs_(min),or 0.3 fs_(min)≤ps_(opt)≤0.5 fs_(min), or 0.4 fs_(min)≤ps_(opt)≤0.5fs_(min).

According to a preferred embodiment the beam pitch size between theindividual beams can be determined for each imaging directionseparately. This means that the beam pitch size between individual beamsin a first direction can differ from the beam pitch size betweenindividual beams in a second direction. Of course, the beam pitch sizebetween individual beams can be the same in both imaging directions.

According to a preferred embodiment, the determined beam pitch size forimaging a respective layer is an optimum beam pitch size. For thisoptimum beam pitch size, the overall time for imaging a layer of the 3Dsamples can be minimized. It is also possible that other imagingconstraints are additionally taken into consideration and that underthese constraints the beam pitch size is the optimum one.

According to the disclosure, the layer of the 3D sample is imaged with asetting of the multi-beam particle microscope based on the determinedpixel size and based on the determined beam pitch size. According to apreferred embodiment, the setting of the multi-beam particle microscopeis exactly the determined pixel size and exactly the determined pitchsize. However, this is not necessarily the case. In any case, thedetermined pixel size and determined beam pitch size are initial valuesfor calculating the setting of the multi-beam particle microscope. It isfor example possible to take an even smaller pixel size for the setting,for example to be on the safe side to carry out imaging with the desiredresolution or for other reasons. On the other hand, it is possible thatusing a larger pixel size than the determined pixel size might beadvantageous, for example because just certain feature sizes are ofinterest in the data evaluation. Analogous considerations hold for thesetting of the multi-beam particle microscope with respect to the beampitch size.

According to a preferred embodiment of the present disclosure, thenumber of pixels in the single field of view is kept constant forimaging a plurality of layers. It has to be stressed that this alsoholds if the resolution for imaging a respective layer changes. In otherwords, the pixel size determined based on feature sizes in therespective layer can change, but the number of pixels in a single fieldof view scanned by an individual charged particle beam is neverthelesskept constant. According to a preferred embodiment, the number of pixelsin a single field of view is kept constant for imaging all layers of the3D sample. A typical number of pixels in the single field of view is3.000×2000=6.000.000 pixels. Typical pixel sizes ps are in the followingrange: 0.5 nm to 20 nm. Then, assuming that in this example the numberof pixels is kept at 3000×2000 pixels, the corresponding beam pitchsizes bps are in the following range: 0.5 μm to 60 μm. Preferably, smalloverlap regions between adjacent single fields of view are created whichfacilitates reconstruction of the entire image and in particularstitching. Advantageously, 5% to 10% of the size of the single field ofview overlaps with a neighboring single field of view in each scanningdimension.

An optimum beam pitch size can be determined in dependence on thedetermined pixel size and the selected number of pixels per single fieldof view scanned by an individual charged particle beam so that adjacentsingle fields of views have the desired overlap, preferably in the rangebetween 5% and 10% of the size of the single field of view in eachscanning direction.

According to a preferred embodiment of the disclosure, the beam pitchvalues in the setting of the multi-beam particle microscope are chosenfrom a set of discrete values. This facilitates calibration of themulti-beam particle microscope. According to another preferredembodiment, the determined beam pitch size is then rounded down to thenext discrete value for the beam pitch size.

According to a preferred embodiment of the disclosure, determining thefeature size in a respective layer is based on a priori knowledge. Itis, for example, possible that the structure of the 3D sample is alreadyknown from its design. This knowledge can be used to make an educatedguess. It is also possible that a basically identical 3D sample of abatch of identical samples was measured before.

According to a preferred embodiment of the disclosure, determining thefeature size in a predefined layer includes imaging of this layer with alight microscope. From a light microscopic overview image, structuresizes in this layer can be inferred. The resolution of a lightmicroscope is lower than the resolution of a particle microscope, andnormally the minimum structure sizes cannot be exactly determined in theimage taken with a light microscope; however, an educated guess forminimum feature sizes can be made. It is possible that only certainareas of the layer to be scanned are imaged with the light microscope.It is also possible to take a complete image with the help of a lightmicroscope, wherein the complete image can optionally be build up by aplurality of individual light microscopic images.

According to another preferred embodiment, determining the feature sizein a respective layer includes taking one or more test images with themulti-beam particle microscope. It is possible to take test images withdifferent pixel sizes and therefore varying resolution. It can then bedetermined whether the smallest feature sizes in the test region can beimaged with the desired accuracy/resolution. The test images can betaken for example in an area that is of specific interest. However, theamount of test images should be limited to an amount that does not slowdown the overall process of imaging the 3D sample in its entirety to anunacceptable degree.

According to an alternative embodiment of the disclosure, determiningthe feature sizes in a respective layer includes carrying out ananalysis of scattered light or scattered particles scattered at thesurface of the 3D sample, respectively. The scattering characteristicsof light or particles are related to the feature sizes of the featuresbeing the origin for scattering.

It is also possible to combine two or more of the above describedmethods for feature size analysis. The pre-requisite is that the featuresize analysis can be carried out comparatively fast and in particularfast enough compared to the overall process of imaging a complete 3Dsample with the multi-beam particle microscope. Preferably, the time forfeature size determination is ≤10%, more preferably ≤1% of the timeinvolved for imaging the 3D sample with the multi-beam particlemicroscope.

According to a preferred embodiment of the present disclosure, themethod further includes the following step: Classifying a respectivelayer into a plurality of regions based on feature sizes in theseregions. Such a classification makes particularly sense when the featuresizes show rather significant variations when comparing differentregions of one layer. It is for example possible that a region in alayer exists that includes only features with large feature sizes, thatanother region exists where the feature sizes are of medium featuresize, and that a third region exists wherein the feature sizes aresignificantly smaller than in the other regions. Therefore, theresolution that is involved to adequately scan the respective layervaries between regions in the respective layer. Preferably, classifyingincludes determining the feature sizes in a layer and dissecting thelayer into regions with similar feature sizes. The feature size,preferably the minimum feature size, is determined for each region.

A single field of view (sFOV) is the area of a sample that is scanned(imaged) with an individual beam. A multiple field of view (mFOV) is thearea that is imaged with a plurality of individual beams simultaneouslyand can include, for example, 20, 50, 100 or 61 or 91 sFOVs (accordingto the general formula 3n (n−1)+1, wherein n is a natural number) oreven more sFOVs. Preferably, a region is set up by a plurality of mFOVs,wherein the minimum feature size in each mFOV is similar, e.g. in aspecified range. Preferably, the plurality of mFOVs forming the regionis neighbored to one another so that the set-up region is mathematicallyconnected. Depending on the minimum feature size in a region, imagingparameters are set for the respective region. Imaging parameters fordifferently classified regions normally vary from one another dependingon the feature sizes assigned to the regions.

The number of regions into which a layer can be classified can vary. Itis possible that 1, 2, 3, 4, 5, 10, 50 or 100 regions or even moreregions can be identified in a layer to be scanned. It is also possiblethat a specific region in a respective layer is of no interest at alland that therefore this region will not be scanned at all. Furthermore,it is possible that a plurality of regions basically exhibits the samefeature size, but that these regions are not connected to each other. Itis also possible that the regions show a certain pattern with respect tofeature sizes in the respective layer.

According to a preferred embodiment the method is characterized in that:determining a feature size of features included in a respective layer iscarried out per region; determining a pixel size based on the determinefeature size in the respective layer is carried out per region;determining a beam pitch size between individual beams in the respectivelayer based on the determined pixel size is carried out per region; andimaging a respective layer of the 3D sample is carried out with asetting of the multi-beam particle microscope per region based on thedetermine pixel size per region and based on the determined beam pitchsize per region.

Here, the setting of the multi-beam particle microscope is not just setper layer, but it is set per region in the respective layer. Since theregions are classified based on feature sizes and the feature sizesinfluence desired resolution and therefore pixel size, the setting ofthe microscope per region within a respective layer allows for an evenbetter speed up and optimization of the entire imaging process.

According to a preferred embodiment of the disclosure, the number ofregions within a respective layer and/or the position of the regionswithin a respective layer vary from layer to layer for a plurality oflayers. Alternatively, according to another preferred embodiment, thenumber of regions within a respective layer and/or the position or theregions within a respective layer is constant for a plurality of layers.When the number of regions and/or the position of the regions within arespective layer varies from layer to layer, the pixel size and beampitch size can be adjusted in a very detailed way in each layer.However, this involves concrete knowledge about feature sizes in eachregion of each layer to be imaged with a high accuracy. Cases also existwherein a priori knowledge yields that the number and position of theregions does not change between the layers, for example because the 3Dsample has a very regular structure that is in principle known. Then, itis an option to define the regions once and to keep this definition foreach layer within the entire imaging process of the 3D sample. Then, thenumber of regions and their position is the same in each layer.

According to another preferred embodiment of the disclosure, the methodincludes quasi non-destructive delayering of the 3D sample. Preferably,a series of ultrathin sections is generated with an ultramicrotome andplaced on a solid substrate. The series of sections is subsequentlyimaged. In principle, other non-destructive delayering techniques arealso possible. Non-destructive in this sense means that the sample isalmost completely available physically after cutting, neglecting thesmall cutting losses.

According to another preferred embodiment of the disclosure, the methodincludes destructive delayering of the 3D sample. According to thisembodiment, the surface of the 3D sample is imaged, and then typically afew nanometers of the surface are removed, with, for example, an ionbeam, and then the freshly exposed surface provides the next layer andis imaged again with the multi-beam particle microscope. Thisdestructive delayering procedure can be repeated many times until acomplete 3D data set of the 3D sample has been acquired. According to apreferred embodiment, the delayering includes ion beam milling. Thisdelayering method is particularly useful when the 3D sample is anintegrated circuit.

According to a second aspect of the disclosure, the disclosure isdirected to a method of imaging a 2D sample with a multi-beam particlemicroscope, the method including: providing a 2D sample; classifying the2D sample into a plurality of regions based on feature sizes in theseregions; determining a feature size, in particular a minimum featuresize, per region; determining a pixel size based on the determinedfeature size per region; determining a beam pitch size betweenindividual beams based on the determined pixel size per region; andimaging the 2D sample with a setting of the multi-beam particlemicroscope per region based on the determined pixel size per region andbased on the determined beam pitch size per region.

A basic difference of the disclosure according to this second aspectcompared to the first aspect of the disclosure is that the sample is a2D sample and not a 3D sample. The 2D sample has just one layer and thesurface of the 2D sample is imaged. Apart from this difference, themethod steps according to this second aspect have already been describedin connection with the first aspect of the disclosure. The definitions,examples and explanations given with respect to the first aspect of thedisclosure also hold for the second aspect of the disclosure; preferredembodiments can also be transferred straight forward from the firstaspect of the disclosure to the second aspect of the disclosure as longas it is no prerequisite that several layers of a sample are imaged.

According to a third aspect of the disclosure, the disclosure isdirected to a computer program product with a program code for executingthe method according to anyone of the embodiments described above. Thecode can be written in any possible programming language and can beexecuted on a control computer system. The control computer system assuch can include one or more computers or processing systems.

According to a fourth aspect of the disclosure, the disclosure isdirected to a system, including:

-   -   a multi-beam particle microscope, including: a charged particle        source configured to generate a first charged particle beam; a        multi beam generator configured to generate a plurality of        charged particle beamlets from an incoming first charged        particle beam, wherein each individual beamlet of the plurality        of charged particle beamlets is spatially separated from other        beamlets of the plurality of charged particle beamlets, an        objective lens configured to focus incoming charged particle        beamlets in a first plane in a manner that a first region in        which a first individual beamlet of the plurality of charged        particle beamlets impinges in the first plane is spatially        separated from a second region in which a second individual        beamlet of the plurality of charged particle beamlets impinges        in the first plane; a projection system and a detector system        including a plurality of individual detectors, wherein the        projection system is configured to image interaction products        leaving the first region within the first plane due to impinging        charged particles onto a first one of the plurality of        individual detectors and to image interaction products leaving        the second region in the first plane due to impinging charged        particles onto a second one of the plurality of individual        detectors; and—a computer system for controlling the multi-beam        particle microscope; wherein the system including the multi-beam        particle microscope and the computer system is adapted to carry        out the method according to any one of the embodiments described        above.

The multi-beam particle microscope can be, for example, a scanningmulti-beam electron microscope (mSEM).

It is possible to combine the described embodiments of the disclosurewith one another as long as no technical contradictions occur.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be more fully understood with reference to theattached drawings, in which:

FIG. 1 is a sketch of an embodiment of a multi-beam charged particlesystem;

FIG. 2 is a sketch of a cross section of an integrated circuit;

FIG. 3 is a diagram illustrating a relation between throughput and pixelsize;

FIG. 4 is a sketch illustrating a definition of pixel size and beampitch;

FIG. 5 is a flow chart of a method of imaging a 3D sample according toan embodiment of the disclosure;

FIG. 6 is a flow chart of a method of imaging a 3D sample according toanother embodiment of the disclosure; and

FIG. 7 is a sketch illustrating regions in a layer classified accordingto different feature sizes.

DETAILED DESCRIPTION

FIG. 1 is a sketch of a particle beam system 1 which employs multipleparticle beams. The particle beam system 1 generates multiple particlebeams which are incident onto an object to be inspected in order to makeelectrons emanate from the object and subsequently detect them. Theparticle beam system 1 is of the scanning electron microscope type (SEM)which employs a plurality of primary electron beams 3 which are incidentat locations 5 on a surface of the object 7 where they generate aplurality of electron beam spots. The object 7 to be inspected can be ofany desired sort and, for example, include a semiconductor wafer, abiological or materials sample and an arrangement of miniaturizedelements or the like. The surface of the object 7 is arranged in anobject plane 101 of an objective lens 102 of an objective lens system100.

The enlarged section I₁ of FIG. 1 shows a top view of the object plane101 with a regular rectangular array 103 of locations of incidence 5which are formed in the plane 101. The number of the locations ofincidence in FIG. 1 is 25, and they form a 5×5 array 103. The number 25of locations of incidence is a small number selected for reasons ofsimplified representation. In practice, the number of beams and/orlocations of incidence can be selected to be much larger—20×30, 100×100and the like, by way of example.

In the embodiment represented, the array 103 of locations of incidence 5is a substantially regular rectangular array with a constant distance P₁between neighboring locations of incidence. This distance P₁ alsoillustrates the beam pitch size which will be described in more detailbelow. Exemplary values of the distance P₁ are 1 micrometer, 10micrometers and 40 micrometers. However, it is also possible for thearray 103 to have other symmetries such as, for example, a hexagonalsymmetry.

A diameter of the beams spots formed in the object plane 101 can besmall. Examples of values of the diameter are 1 nanometer, 5 nanometers,100 nanometers and 200 nanometers. The focusing of the particle beams 3for the formation of the beam spots is performed by the objective lenssystem 100.

The particles incident onto the object, generate electrons which emanatefrom the surface of the object 7. The electrons emanating from thesurface of the object 7 are formed into electron beams 9 by theobjective lens 102. The inspection system 1 provides an electron beampath 11 for feeding the multiplicity of electron beams 9 to a detectionsystem 200. The detection system 200 includes electron optics with aprojection lens 205 for directing the electron beams 9 onto an electronmulti-detector 209.

Section I₂ in FIG. 1 shows a top view of a plane 211 in which individualdetection regions are lying onto which the electron beams 9 are incidentat certain locations 213. The locations of incidence 213 lie in an array217 at a regular distance P₂ from one another. Exemplary values of thedistance P₂ are 10 micrometers, 100 micrometers and 200 micrometers.

The primary electron beams 3 are generated in a beam generating device300 which includes at least one electron source 301, at least onecollimation lens 303, a multi-aperture arrangement 305 and a field lens307. The electron source 301 generates a diverging electron beam 309which is collimated by the collimation lens 303 in order to form a beam311 which illuminates the multi-aperture arrangement 305.

The section I₃ in FIG. 1 shows a top view of the multi-aperturearrangement 305. The multi-aperture arrangement 305 includes amulti-aperture plate 313 which has a plurality of openings or apertures315 formed therein. The centers 317 of the openings 315 are arranged inan array 319 which corresponds to the array 103 which is formed by thebeam spots 5 in the object plane 101. A distance P₃ of the centers 317of the apertures 315 from one another can have, for example, values of 5micrometers, 100 micrometers and 200 micrometers. The diameters D of theapertures 315 are smaller than the distance P₃ of the centers of theapertures. Exemplary values of the diameters D are 0.2×P₃, 0.4×P₃ and0.8×P₃.

Electrons of the illuminating beam 311 penetrate the apertures 315 andform electron beams 3. Electrons of the illuminating beam 311, which areincident onto the plate 313, are captured by the latter, and do notcontribute to formation of the electron beams 3.

Owing to an imposed electrostatic field, the multi-aperture arrangement305 focuses the electron beams 3 in such a way that beam foci 323 areformed in a plane 325. A diameter of the foci 323 can be 10 nanometers,100 nanometers and 1 micrometer, for example. The field lens 307 and theobjective lens 102 provide a first imaging particle optics for thepurpose of imaging the plane 325, in which the foci are formed, onto theobject plane 101 so as to form the array 103 of locations of incidence 5or beam spots on the surface of the object 7. The objective lens 102 andthe projection lens 205 provide a second imaging particle optics for thepurpose of imaging the object plane 101 onto the detection plane 211.The objective lens 102 is therefore a lens which is both part of thefirst and of the second particle optics, while the field lens 307belongs only to the first particle optics, and the projection lens 205belongs only to the second particle optics.

A beam switch 400 is arranged in the beam path of the first particleoptics between the multi-aperture arrangement 305 and the objective lenssystem 100. The beam switch 400 is also part of the second particleoptics in the beam path between the objective lens system 100 and thedetection system 200.

Further information relating to such multi-beam inspection systems andcomponents employed therein such as, for example, particle sources,multi-aperture plates and lenses, can be obtained from the InternationalPatent Applications WO 2005/024881, WO 2007/028595, WO 2007/028596 andWO 2007/060017 and the German patent applications with the applicationnumbers DE 10 2013 016 113.4 and DE 10 2013 014 976.2, the content ofdisclosure of which is incorporated in full in the present applicationby reference.

The depicted exemplary particle beam system 1 also includes a computersystem 10. This computer system 10 can include several computers and/orsub computer systems. It can include for example a control computersystem for controlling the particle beam system 1, one or more imageacquisition systems and a user interface. Other configurations are alsopossible.

The computer system 10, or the components of the computer system 10serving for assembling the detected data to an image, includes at leastone frame grabber. This frame grabber also obtains information about howfar the scanning deflection of the particle beams 3 incident on theobject 7 has advanced. By way of example, this information can be fed tothe frame grabber via a clock signal, which is output by the computersystem 10 or a different clock generator and, for example, likewiseserves for controlling the scanning deflection of the particle beams 3.The frame grabber then respectively generates image information byvirtue of integrating particle intensities detected while scanning overa certain distance on the object, and converting them into greyscalevalues of an image and assigning these to a location in the image. Thepixel size is then defined and adjusted as the distance on the objectover which the particle intensities are integrated and assigned a singlelocation (pixel) in the image.

FIG. 2 is a sketch of a cross-section of an integrated circuit. Today'sintegrated circuits can contain up to 15 or more layers with structures.The integrated circuit shown in FIG. 2 represents a schematic example ofan integrated circuit in a very simplified way. The depicted integratedcircuit includes five layers in z-direction. The first layer on topshows comparatively large features. The second layer as counted inz-direction includes smaller features when compared to the top layer.The third layer as counted from the top in z-direction includes featureswith a smaller feature size compared to the features included in thesecond layer etc. In general, the feature sizes gradually decrease fromlayer to layer in z-direction. The bottom layer of the integratedcircuit includes the smallest features.

When imaging an integrated circuit with a plurality of layers, it isuseful to also image intermediate layers to make sure that theinformation about the structure gained by the imaging is complete. Forexample, it might be advantageous to image 30 layers if the integratedcircuit includes 15 structure layers. Of course, it is also possible toimage even more layers/intermediate layers.

For imaging small features included in a layer, it is desirable to carryout imaging with a comparatively high resolution. Therefore, the pixelsize has to be chosen comparatively small during scanning.Advantageously, an optimum pixel size can be chosen based on the minimumfeature size present in the respective layer. Advantageously, thefollowing relation holds between the minimum feature size fs_(min) andthe optimum pixel size ps_(opt): ps_(opt)≤0.5 fs_(m)m. Preferably, atleast one of the following relations is fulfilled: 0.1fs_(min)≤ps_(opt)≤0.5 fs_(min), or 0.2 fs_(min)≤ps_(opt)≤0.5 fs_(min),or 0.3 fs_(min)≤ps_(opt)≤0.5 fs_(min), or 0.4 fs_(min)≤ps_(opt)≤0.5fs_(min).

FIG. 3 is a diagram illustrating a relation between throughput and pixelsize, but without adjustment of beam pitch size. The throughput isindicated in arbitrary units and the pixel size is provided innanometers. The three graphs included in FIG. 3 show the relationbetween pixel size and throughput for a slow speed stage (straightline), for a medium speed stage (broken line) and for a fast speed stage(dotted line). If the pixel size is enlarged, the throughput generallyincreases. This effect is generally known from single beam particlemicroscopes. However, when a certain pixel size is reached (in the shownexample this is already the case with pixel sizes of about 3 to 4nanometers), the total data acquisition time starts to get dominated byoverhead times, such as time for autofocusing/time for autostigmationand time for stage movements which do not scale with pixel size. Theshown throughput calculation clearly shows that a high throughput isonly achieved with a fast speed stage. The disclosure provides asolution to arrive at a high throughput also for a medium speed stageand even for a slow speed stage.

The key feature of this solution is to also adapt the beam pitch sizebetween the individual beams in a respective layer that is imaged basedon the determined pixel size. Preferably, the beam pitch size isoptimized.

FIG. 4 is a sketch illustrating a definition of pixel size and beampitch size according to the present disclosure. In FIG. 4 , only threebeams b1, b2 and b3 are shown for simplicity. Today's multi-beam chargedparticle microscopes normally include many more individual beams, forexample more than 50, more than 80, or more than 100 beams. The beams assuch can be arranged, for example, in a rectangular pattern. Otherarrangements are also possible. Preferably, the individual beams arearranged according to a hexagonal structure, for example with 61 or 91individual beams (according to the general formula 3n (n−1)+1, wherein nis a natural number).

In FIG. 4 , each individual beam b1, b2 and b3 scans its particularsingle field of view (sFOV). This sFOV includes a certain number ofpixels which are indicated for individual beam b1 with a rectangularpattern of dotted lines. It is noted that, in principle, the completesFOV is scanned with the respective individual beam b1, b2 and b3, butFIG. 1 shows just an excerpt of all pixels for grounds of simplicity.The pixel size ps indicates the lateral increment of beam positionduring the imaging process between two neighbored pixels of the image,or with other words, the pixel size ps indicates the lateral incrementof the beam positions during the scanning of the sample layer by theplurality of individual charged particle beams. In the depicted example,the pixel size ps is equal in directions x and y. However, it is alsopossible that the pixel size ps in x direction differs from the pixelsize ps in y direction.

The beam pitch size bps indicates the distance between the individualbeams during the scanning of the sample layer by the plurality ofindividual charged particle beams. In FIG. 4 , the beam pitch size bpsis illustrated as the distance between the centers of the sFOVs of beamb1 and beam b2. The beam pitch size bps in FIG. 4 is indicated for thex-direction. The beam pitch size bps can have the same value or adifferent value in y-direction. Preferably, the beam pitch size bps isthe same for all neighbored pairs of individual beams which can e.g. beachieved with a hexagonal or quadratic arrangement of the individualbeams. For other arrangements, e.g. a rectangular arrangement, the beampitch size can be different in different directions. In general,different imaging directions can be orthogonal to each other, but thisis not necessarily the case.

FIG. 5 is a flow chart of a method of imaging a 3D sample according toan embodiment of the disclosure. According to method step S1, layer n ofthe 3D sample is provided. In principle, it is possible that the 3Dsample is delayered in a destructive or in a non-destructive manner. Ifa non-destructive approach is used, the 3D sample can be cut or slicedinto a plurality of layers, each layer is put onto a substrate and canthen be individually investigated with the multi-beam particlemicroscope. If the destructive delayering approach is applied, thesurface of the 3D sample is preferably imaged first and afterwards thissurface is removed so that the new surface generated by delayeringembodies the next layer that can be scanned. A possible technicalrealization is, for example, ion beam milling.

In step S2 a feature size of features included in layer n is determined.Preferably, a minimum feature size present in this layer n isdetermined. Advantageously, a fast method is applied for determining thefeature size. Here, fast means comparatively fast if compared to theimaging time to image a complete layer of the 3D sample with themulti-beam particle microscope. Preferably, a fast method involves ≤10%and more preferably ≤1% of the time that is desired for imaging thesample with the multi-beam particle microscope. Alternatively, it isalso possible to determine the minimum feature size in the respectivelayer n based on an educated guess, for example if the feature size isknown by design. Fast methods for determining the feature size, inparticular the minimum feature size, in a respective layer include, butare not limited to the following measurement methods: imaging layer nwith a light microscope, taking one or more test images with themulti-beam particle microscope, and carrying out an analysis ofscattered light or scattered particles. The determined feature size, inparticular the minimum feature size, can be determined as an absolutevalue.

Then, in step S3, a pixel size is determined based on the determinedfeature size in layer n. Preferably, the determined pixel size is anoptimum pixel size. Then, the chosen pixel size is still small enough toadequately image the smallest features included in the layer.Preferably, the optimum pixel size is equal to or smaller than half ofthe minimum feature size in the respective layer.

In step S4, a beam pitch size between the individual beams of themulti-beam charged particle microscope in layer n is determined based onthe determined pixel size. Preferably, the beam pitch size is an optimumbeam pitch size. The throughput achieved with the optimum pixel size andthe optimum beam pitch size is preferably as high as possible for agiven stage speed. Preferably, the number of pixels in the single fieldsof view is kept as large as possible which leads to a large mFOV aswell. However, as the size of the mFOV increases, the aberrations of theelectron-optical system in the particle-beam microscope will alsoincrease and give rise to a larger size of the beam spots 5 on thesample. The beam spot size determines the resolution that can beobtained by the multi-beam particle microscope and should preferably besmaller than the pixel size. Therefore, the size of the mFOV cannot bechosen arbitrarily large and this also limits the number of pixels thatcan be adequately used. According to an alternative embodiment, thenumber of pixels acquired with an individual beam is kept constant. Interms of calibration, it is advantageous if the beam pitch size ischosen from a set of discrete values like for example 12 μm, 15 μm etc.Then, it is preferable that the determined beam pitch size is roundeddown to the next discrete value in the set. Furthermore, it ispreferable that the beam pitch size is chosen such that there exists asmall overlap between neighbored sFOVs to ensure an adequate overallimage reconstruction (stitching). In practice, 1% to 15% of the overallarea of the sFOV overlap with neighbored sFOVs. Of course, other valuesfor an overlap between sFOVs can be chosen.

In the next step S5, the charged particle optical components of themulti-beam charged particle microscope are adjusted to achieve thedetermined beam pitch size between the individual beams of themulti-beam charged particle microscope. This adjustment can be achievedby adjusting the combined excitations of the field lens 307 and theobjective lens 102 in the embodiment shown in FIG. 1 , i.e. by adjustingthe magnification or imaging scale with which the plane 325 (in whichthe individual beamlets 323 are focused) is imaged into the plane 101.In addition the scanning ranges of the scanning deflectors (not shown inFIG. 1 ) are adjusted in a manner that each single field of view has thedesired dimensions in both scanning directions.

As described above with reference to FIG. 1 , the computer system 10, orthe components of the computer system 10 serving for assembling thedetected data to an image includes at least one frame grabber. The pixelsize is defined and adjusted as the distance on the object over whichthe particle intensities are integrated and assigned a single location(pixel) in the image by the frame grabber.

Thereafter, layer n of the 3D sample is imaged with the setting of themulti-beam particle microscope based on the determined pixel size andbased on the determined beam pitch size. Subsequently, the next layern+1 is provided (either already prepared or by delayering the 3D samplein a destructive way). Then, method steps S2, S3, S4 and S5 are carriedout again. The overall method can be carried out repeatedly until everylayer of the 3D sample that is of interest is scanned.

FIG. 6 shows a flow chart of a method of imaging according to anotherembodiment of the disclosure. The difference between the embodimentdepicted in FIG. 6 compared to the embodiment shown in FIG. 5 is thatafter the layer n is provided in step S1, layer n is classified intoregions in step S6. This classification is carried out based on featuresizes in these regions. If the feature size significantly varies in alayer, a classification into regions makes sense to individually adaptthe imaging parameters. This means for example, that the minimum featuresize of a region is determined, that subsequently the optimum pixel sizeis determined based on the determined minimum feature size in thisregion, and that furthermore the beam pitch size is also determined perregion. In this way, adjustment of setting parameters of the multi-beamparticle microscope is carried out for every region or at least forevery region that is of interest. If a region does not include featuresof interest, it is not necessary to set measurement parameters for thisregion or to even measure this region with the multi-beam particlemicroscope.

In general, a region with small feature sizes involves as a setting acomparatively small pixel size and also a comparatively small beam pitchsize. A region with a medium feature size basically involves as asetting a medium pixel size and a medium beam pitch size. A region withlarge feature sizes basically involves a setting with a large pixel sizeand with a comparatively large beam pitch size. If a region does notinclude any relevant features, no imaging needs to be carried out.

After layer n is imaged (fully or just partly), the next layer n+1 isprovided, either by non-destructive delayering or by destructivedelayering of the 3D sample, for example by ion beam milling.

FIG. 7 is a sketch illustrating regions in a layer classified bydifferent feature sizes. In the depicted example three different regionswith different feature sizes (region A, region B and region C) areshown. When defining regions A, B and C, reference can be made tocharacteristic feature size ranges. It is possible to determine featuresizes in a layer with a fast method first and then to define the regionsbased on typical feature sizes, respectively. It is also possible topre-define feature size ranges and then to assign the regions in arespective layer to this feature size range.

Region A includes comparatively large features and therefore this regioncan be imaged with a setting characterized by a comparatively largepixel size and as well by a comparatively large beam pitch size. In thedepicted example, seven mFOVs indicated by the big hexagons are used.The hexagonal structure is the result of a hexagonal arrangement of theplurality of beams of the multi-beam particle microscope according to apreferred embodiment. However, other beam arrangements like for examplea rectangular arrangement are also possible. Region B includescomparatively small features and here the resolution has been chosencomparatively high. In other words, the pixel size is comparativelysmall and so is the beam pitch size. This is illustrated by the smallhexagons. Region C includes features in a medium feature size range.Here, the setting parameters for imaging include a medium pixel size andalso a medium beam pitch size. In FIG. 7 , this is indicated by themedium size hexagons.

Adjusting the setting of a multi-beam particle microscope for each layerand furthermore for specific regions in the respective layer allows fora very accurate and fast imaging process. In particular, it becomespossible to significantly lower the time needed to fully analyze a 3Dsample.

What is claimed is:
 1. A method, comprising: determining a pixel sizebased on a feature size in a 2D sample; determining a beam pitch sizebetween individual beams of a multi-beam particle microscope based onthe determined pixel size; and using the multi-beam particle microscopeto image the 2D sample with a setting of the multi-beam particlemicroscope based on the determined pixel size and based on thedetermined beam pitch size.
 2. The method of claim 1, wherein thefeature size is a minimum feature size in the 2D sample.
 3. The methodof claim 2, wherein the pixel size is at most half of the minimumfeature size.
 4. The method of claim 1, wherein, for a first singlefield of view, single fields adjacent to the first single field of viewoverlap with the first single field of view by between 5% and 10% ineach scanning direction of the multi-beam particle microscope.
 5. Themethod of claim 1, further comprising selecting the beam pitch size froma set of discrete values of the multi-beam particle microscope.
 6. Themethod of claim 5, further comprising rounding down the determined beampitch size to the next discrete value for the beam pitch size.
 7. Themethod of claim 1, further comprising using a priori knowledge todetermine the feature size in the 2D sample.
 8. The method of claim 1,further comprising using a light microscope to image the 2D sample todetermine the feature size of the 2D sample.
 9. The method of claim 1,further comprising using the multi-beam particle microscope to take atleast one test image to determine the feature size in the layer of the2D sample.
 10. The method of claim 1, further comprising analyzingscattered light or scattered particles to determine the feature size inthe 2D sample.
 11. The method of claim 1, further comprising classifyingthe 2D sample into a plurality of regions based on feature sizes in theregions.
 12. The method of claim 11, further comprising: determining,per region, a feature size of features in the 2D sample; determining,per region, a pixel size based on the determined feature size in theregion; determining, per region, a beam pitch size between individualbeams of the multi-beam particle microscope; and imaging, per region,the 2D sample with a setting of the multi-beam particle microscope basedon the determined pixel size in the region and based on the determinedbeam pitch size for the region.
 13. One or more machine-readablehardware storage devices comprising instructions that are executable byone or more processing devices to perform operations comprising themethod of claim
 1. 14. A system comprising: one or more processingdevices; and one or more machine-readable hardware storage devicescomprising instructions that are executable by the one or moreprocessing devices to perform operations comprising the method ofclaim
 1. 15. The system of claim 14, further comprising a multi-beamparticle microscope.